This invention relates to a circuit control means to work with a thermal resistive type fluid level sensor. Although a thermistor type sensor could be used, the preferred embodiment uses a liquid level sensor which employs semiconductive material which, over a temperature range, exhibits a peak electrical resistance at a predetermined temperature and predetermined varying resistance over the rest of the temperature range. Such a device is further described in U.S. Pat. No. 4,065,760.
The subject invention is employed in an automotive engine environment to detect the level of engine oil. The invention provides for constant current excitation to the liquid level sensor. The constant current excitation is shut off under a no-oil condition thereby providing a no-oil signal to some type of message center while also protecting the oil level sensor from damage under this high temperature self-heat condition. Another advantage of utilizing the constant current excitation is that the response from the oil level sensor is very quick because maximum power can be delivered to the sensor.
If constant voltage excitation were utilized, a resistor would be placed in series with the sensor to limit the current drawn to that value of the sensor's maximum allowable power. The sensor exhibits peak resistance at a mid-range temperature value and exhibits minimal resistance at both low and high temperature values. Under a constant voltage excitation condition, the sensor, when operating at low temperatures, cannot heat up quickly enough to respond properly; this is due to the current limiting, resistive-drive excitation. The current draw at low temperatures will not sufficiently heat up the sensor by self-heating. At high temperatures when the sensor is excited with constant voltage, the current through the sensor is greater due to the device's decreased resistance at high temperatures. Therefore, the high temperature current rating limits the amount of current that you can safely source through the device with a limiting resistor. This limits the sensor, at lower temperatures where the device exhibits higher resistance, to a lower current draw than required for proper self-heating action and response.
If you drive the sensor with a constant current device, you can dissipate more power within the device and therefore operate it faster. With constant current excitation, a current value is selected to operate the sensor near the high temperature power limit at all times (with appropriate automatic over-temperature protection). This condition provides for the quick response from the sensor required (it is estimated that the response is in order of magnitude better than with constant voltage excitation). Constant current excitation also minimizes the effects of the voltage fluctuations typically experienced in automotive applications.